1. Field of the Invention
The present invention relates to a semiconductor device. The present invention relates to an image display device. The present invention relates to a storage device.
In this specification, a semiconductor device means all types of devices that can function by utilizing semiconductor characteristics, and a transistor, a semiconductor circuit, a storage device, an imaging device, a display device, an electro-optical device, an electronic device, and the like are all embodiments of semiconductor devices.
2. Description of the Related Art
Semiconductor devices that include a plurality of functional circuits including selection transistors are known. The semiconductor devices are applied to image display devices including a liquid crystal element, an electroluminescent (EL) element, or an electrophoretic element, storage devices including storage elements such as a dynamic random access memory (DRAM) element and a static random access memory (SRAM) element, or the like.
For example, as a display device to which a selection transistor is applied, a display panel using an organic EL medium is disclosed in Patent Document 1.
As the selection transistor, an n-channel transistor is often used. Some of the reasons are as follows: an n-channel transistor can operate at high speed because electrons with high mobility are used as carriers, and is suitable for miniaturization because large current can flow through even a relatively small n-channel transistor.
In recent years, small portable devices such as a mobile phone and a tablet terminal have been developed. Since these devices are powered by a battery, lower power consumption has been desired. In order to make the devices smaller, it has been also desired to reduce the number of circuit elements included in the devices and wirings for connecting the circuit elements as much as possible.
Further, an increased drive frequency of the semiconductor devices has been desired. In an image display device, for example, in order to improve its display quality, it has been examined to increase the number of pixels or perform high-speed display (e.g., at double speed or quad speed). In a storage device, for example, high-speed data writing or data reading has been required.